As an editor I want to be able to merge two Lexemes in order to eliminate duplicates.
**Screenshots:**
Before merge of two items:
{F22636219}
After successful merge of two items:
{F22636079}
**BDD**
```
GIVEN I have two Lexemes that represent the same word
WHEN I go to Special:MergeLexemes
AND enter their IDs
AND I click "merge Lexemes"
THEN the source Lexeme is emptied and replaced by a redirect to the target Lexeme
AND the lemmas are merged
- lemmas that exist both on the target and the source are kept on the target
- lemmas that only on the source are added to the target
AND the statements are merged
- statements that exist both on the target and the source are kept on the target
- statements that exist only on the source are added to the target
AND the forms are merged
- if there exists a form on the source that is identical to a form on the target (i.e. they have identical grammatical features, at least one common representation and no different representations for the same spelling variant)
- their representations are merged in the same way lemmas are merged
- their statements are merged in the same way they're merged on the lexeme level
- forms that only exist on the source are added to the target
AND I get a confirmation message saying "L123 was merged into L1234 and redirected."
```
**Acceptance criteria:**
* A new SpecialPage Special:MergeLexemes exists and can be used to merge two Lexemes.
* After a merge the source Lexeme contains a MediaWiki redirect to the target Lexeme (just like it is done for items).
* After the merge I can go to the URL of the source Lexeme and am redirected to the target Lexeme.
* If there are identical Forms (same representation and grammatical features) in each Lexeme then the statements for that Form get added to the existing Form.
**Notes:**
* Blocking of specific bad merges is not covered in this ticket.
* Redirecting on the Form level is not to be done at this point - only the Lexeme level
* Text and design on Special:MergeLexemes should be the same as on Special:MergeItems with s/Items/Lexemes. The text for the longer message should be "If you merge two Lexemes, all representations, Forms, Senses and statements will be moved from one Lexeme to the other."
**Open questions:**
* What happens on items when both have the exact same statement? Do they get duplicated or does this statement get ignored during the merge?